I'm not sure exactly where you mean for the decrease, so I can't picture it. But I can say that what kind of decrease you use depends on whether you want it to be hidden or visible or decorative.
It sounds like you're making the sweater in front and back pieces that will be sewn together. In that case, the most invisible decrease, I believe, (correct me if I'm wrong, folks) would be a right slanting decrease at the very beginning of the right-side row, and a left slanting decrease at the very end. K2tog, followed by ssk at the end of the row would work fine (on a right side row). Stitch 'N Bitch illustrates this on page 63, picture captioned "right slanting decrease on right edge." If you need to do it on a wrong side row you can use slip slip purl (I'll be putting up a video of this soon, let me know if you need verbal instructions ASAP), followed by p2tog.
As far as which row is the "following 4th row," it sounds like the 9th row--UNLESS you are working in pattern instructions that outline what to do in a repeated series of rows, like over, say, 6 rows. You know, where they tell you:
Row 1: knit
Row 2: purl
...Row 6: purl
Repeat rows 1 - 6.
So, "Row 4" has a designation. Then it may be Row 4 of that series.
Hope that helps!